1 Field of the Invention
The present invention relates to an image processing apparatus for reproducing a high quality of image.
2. Related Background Art
In binarizing a digital image signal to form an image by a laser beam printer, it has been proposed, in order to attain half-tone tonalities, to convert the digital image signal to an analog signal and compare the analog image signal with a reference signal such as a ramp wave signal to produce a pulse-width-modulated binary signal. In such an image processing apparatus, a plurality of ramp waves are generated depending on the types of the digital wave signals and the ramp or triangular waves are selectively used depending on the type of the image signal.
Those methods have been proposed by the assignee of the present invention in U.S. patent applications Ser. No. 765,938 filed on Aug. 15, 1985, Ser. No. 897,053 filed on Aug. 15, 1986, Ser. No. 898,096 filed on Aug. 20, 1986, Ser. No. 900,603 filed on Aug. 26, 1986, Ser. No. 900,998 filed on Aug. 27, 1986. Ser. No. 901,825 filed on Aug. 29, 1986, Ser. No. 919,763 filed on Oct. 16, 1986, Ser. No. 923,026 filed on Oct. 24, 1986, Ser. No. 932,030 filed on Nov. 11, 1986, Ser. No. 010,539 filed on Feb. 3, 1987, Ser. No. 012,541 filed on Feb. 9, 1987, Ser. No. 012,322 filed on Feb. 9, 1987, Ser. No. 013,629 filed on Feb. 11, 1987, and Ser. No. 033,462 filed on Apr. 2, 1987.
However, such a ramp wave signal is affected by other ramp wave signals and can overshoot or undershoot as shown in FIGS. 4A and 4B. As a result, a pulse width of a binarized pattern signal, which is theoretically W as shown in FIG. 5A, is shortened by distortions 60 and 61 of the ramp wave as shown in FIG. 5B.
The distortion of the pulse width of the binarized pattern signal causes reversal of density or poor quality in a critical area when a half-tone image is to be produced. This also causes fatal degradation of image quality, giving rise to such problems as pseudo-outline in a line image.